The invention relates to a resonance absorber comprised of moderately or weakly absorbing optical materials, for example, semiconductor materials, for the conversion of photon energy of electromagnetic radiation energy to other forms of energy for use in semiconductor-photodiodes, photoresistors, waveguide lasers, irreversible optical recording media, photothermal transformers for solar energy, and other physical operating components.
Absorbers of moderately or weakly absorbing materials for incident photons or electromagnetic radiation, respectively, are known in numerous embodiments. Such absorbers often have dimensions within the range of the penetration depth of the photons or the electromagnetic radiation, respectively. The penetration depth is defined as the length of the path along which the number of photons of the intensity of radiation has decreased to the fraction e.sup.-4.pi.. The depth of penetration, XE, becomes x.sub.E =.lambda./k because--as is well known--the radiation intensity I decays in substances having an absorption coefficient k according to the formula ##EQU1## where I.sub.o equals the intensity at point x=0, where x is the length of path described by the radiation and .lambda. the wavelength of the electromagnetic radiation. Due to the reflection occurring at the interfaces of absorbing materials, the radiation impinging on the surface is only incompletely absorbed. Therefore, the known methods for reduction of reflection use a plurality of absorbers so that up to 100% of the incident radiation that enters the absorber is absorbed along the depth of penetration.
The physical processes which are triggered by the photon-absorption, occur in numerous physical processes, although in a portion of the area of the absorbing material that is small compared to the depth of penetration, for instance in the zone in the immediate vicinity of the surface of the absorber. In that case the portion of energy absorbed outside of this zone is lost for the desired process. This effect is very disadvantageous in semiconductor methods for wavelengths having small absorption coefficients and also having large depths of penetration.
Therefore, it is possible to employ absorbing devices of relatively small dimension whose limiting planes are partially reflective as resonance absorbers, so that standing waves in the absorbing material between these planes result. The demands would not be adequately fulfilled by resonance absorbers of moderately or weakly absorbing materials, despite their advantage of having a relatively short absorber length. Even when the reflection of the anterior peripheral plane is considerably reduced externally by anti-reflection methods and when the posterior limits are occluded by a highly reflecting heavy metal layer which prevents the radiative energy from being transmitted through the whole system, the impinging radiation is still only incompletely absorbed within the resonance absorber because a considerable portion of the radiative energy from the resonance absorber enters the metal and is absorbed therein. Thus the radiation does not participate in the desired physical effect in the absorber which determines the function, for instance, of a semiconductor.
The invention provides a resonance absorber of small dimensions, wherein the available radiation or photon energy is used completely, or at least almost completely, for the process of energy transformation in the absorbing material.
The object of the invention is to provide a resonance absorber for the absorption of impinging photons of electromagnetic radiation where the spatial absorption range is reduced to be within a zone of the absorbing material which is technically desirable for the transformation of energy.